The present invention relates to tanks for storing gases and liquids. More particularly, the present invention relates to a method of manufacturing fuel tanks for automobiles and trucks.
It is preferred that automotive fuel tanks be lightweight, and of a sufficient strength and durability to meet automotive safety requirements. In addition, automotive fuel tanks must be able to contend with harsh environmental conditions, and thus must be corrosion resistant.
The fuel tank of an automobile is usually designed in accordance with the design of the body in the final stage, and the shape has tended to become more and more complicated in recent years. Thus, the fuel tank material should have an excellent deep drawability and not crack subsequent to forming. In addition, it is important that the material not corrode so as to lead to pitting corrosion and filter clogging. The material must also be easily and stably welded.
In cost-sensitive applications such as automotive fuel tanks, conventional engineering materials force a trade-off between cost and fuel efficiency, safety, and performance. Simply, a lightweight weak fuel tank compromises the durability of the tank and the safety of the vehicle occupants while a heavy strong fuel tank compromises the cost and fuel efficiency of the vehicle. As graphically depicted in FIG. 1, structural materials are currently available in a broad range of strength-to-weight ratios, or specific strengths, but the costs of these materials generally increase disproportionately to their specific strengths. Carbon composites and titanium, for example, while being perhaps ten times stronger than mild steel for a given weight, are typically more than fifty times more expensive. Consequently, such high performance materials are typically used only in on small items or in applications where the high cost is justified, such as in aircraft.
Automobile fuel tanks have generally been manufactured by plating surfaces of a soft steel sheet with a lead alloy and shaping and welding the coated steel sheet. A Pb—Sn alloy-plated steel sheet, which is called a terne steel sheet, has been used for fuel tanks. The steel sheet has chemical properties stabilized against gasoline, and shows excellent press formability due to the excellent lubricity of the plating. In addition to the Pb—Sn alloy-plated steel sheet, a Zn-plated steel sheet which is thickly chromated has also been used. The steel sheet also has excellent formability and corrosion resistance though not as good as the Pb—Sn alloy-plated steel sheet. However, a material not using Pb is desired from the standpoint of decreasing environmental pollution.
One of the prospective fuel tank materials of automobiles in which Pb is not used is an aluminum (Al—Si) plated steel sheet. Since aluminum forms a stabilized oxidized film on its surface, aluminum provides excellent resistance to corrosion caused by organic acids formed by the deterioration of alcohol, gasoline, etc. However, there are several problems with using the aluminum plated steel sheet as a fuel tank material. Since the aluminum plated steel sheet has a very hard Fe—Al—Si intermetallic compound layer formed at the interface between the plating layer and the steel sheet, the Al-plated steel sheet tends to crack when formed. The aluminum plated steel sheet also has the disadvantage that the peeling of the plating and crack formation tend to take place from a starting point in the alloy layer. When cracks are formed in the plating, corrosion tends to proceed from the cracks, and pitting may result in a short period of time. Accordingly, corrosion resistance subsequent to forming is a serious problem. Another problem is weldability. Although an aluminum plated steel sheet may be resistance welded, the welding lacks stability to some degree.
A stainless steel sheet is a fuel tank material capable of satisfying the requirement for higher corrosion resistance demanded from the standpoint of eliminating unacceptable corrosion. The use of austenitic stainless steels, which requires no lining treatments, has been attempted. Although the austenitic stainless steels exhibit superior processability and higher corrosion resistance compared with the ferritic stainless steels, the austenitic stainless steels are expensive for fuel tanks and have the possibility of stress corrosion cracking (SCC). Thus, the austenitic stainless steels have not yet been used in practice. In contrast, the ferritic stainless steels not containing nickel are advantageous in material costs compared with the austenitic stainless steels, but do not exhibit satisfactory corrosion resistance to so-called “deteriorated gasoline” containing organic acids, such as formic acid and acetic acid, which are formed in the ambient environment. Furthermore, the ferritic stainless steels do not exhibit sufficient processability to deep drawing for forming fuel tanks having complicated shapes.
As reflected in FIGS. 1 and 2, air hardenable martensitic stainless steels have exceptionally strength, particularly compared to common metals such as aluminum and even titanium. Nevertheless, such steels are relatively affordable. Air hardening steels have been commercially employed for use in cutlery for their high hardness. Common air hardenable steels include martensitic stainless steels. As defined herein, and as understood by those skilled in the art, air hardenable martensitic stainless steels are essentially alloys of chromium and carbon that possess a body-centered-cubic (bcc) or body-centered-tetragonal (bct) crystal (martensitic) structure in the hardened condition. They are ferromagnetic and hardenable by heat treatment, and they are generally mildly corrosion resistant.
Air hardenable martensitic stainless steels include a relatively high carbon and chromium content compared to other stainless steels with a carbon content between 0.08% by weight and 0.75% by weight and a chromium content between 11.5% by weight and 18% by weight. As reflected in FIG. 3, air hardenable martensitic stainless steels have also been defined, and are understood by those skilled in the art, as having a nickel equivalent of between about 4 and 12 and having a chromium equivalent of between about 8 and 15.5, where nickel equivalent is equal to (% Ni+30×% C)+(0.5×% Mn) and chromium equivalent is equal to (% Cr+% Mo+(1.5×% Si)+(0.5×% Nb). Either or both of these definitions are acceptable for practicing the present invention. According to these standard definitions, standard air hardenable martensitic stainless steels include types 403, 410, 414, 416, 416Se, 420, 420F, 422, 431, and 440A-C.
The relatively high carbon and chromium content compared to other stainless steels results in steel with good corrosion resistance, due to the protective chromium oxide layer that forms on the surface, and the ability to harden via heat treatment to a high strength condition. Unfortunately, the high carbon and chromium also presents difficulties related to brittleness and cracking in welding, and accordingly martensitic stainless steel has been primarily used for cutting tools, surgical instruments, valve seats, and shears. Non-stainless air hardenable steels, which contain very high levels of carbon to allow the formation of a martensitic microstructure upon quenching, also present difficulties related to brittleness and cracking. In fact, experimentation with air hardenable stainless steels for tank applications, and particularly automotive fuel tank applications, appears to have never been attempted due to the paradigm shift in thinking required to produce a high-strength automotive part. Historically, high-strength automotive applications relied on the evolutionary approach of forming a ferrous alloys strip, in its final metallurgical microstructure, using successively higher strength steels as the raw material until either the strength targets were met or the part could not be formed due to the material's limitations.
The use of air hardenable martensitic stainless steels for golf clubs and bicycle applications was introduced in U.S. Pat. Nos. 5,485,948 and further described in 5,871,140. These patents describe brazed tube structures that take advantage of the fact that air hardenable stainless steel can be simultaneously brazed and hardened in one heat treating operation. However, there is no suggestion as to how to use such a material for tanks for storing liquids or gases such as automotive fuel tanks.
Thus, rather than resort to the use of expensive alloys, it would be beneficial to create a process that could utilize a common inexpensive air hardenable stainless steel to produce storage tanks substantially free of cracks. Such a process would be even more beneficial if the material possessed the corrosion resistant properties of stainless steel.
Furthermore, it would be desirable for an improved method for manufacturing automotive fuel tanks which are built strong and lightweight, yet are produced at a low costs.